Conversation assistance devices aim to make conversations more intelligible and easier to understand. These devices aim to reduce unwanted background noise and reverberation. One path toward this goal concerns linear, time-invariant beamforming with a head-mounted microphone array. Application of linear beamforming to conversation assistance is, in general, not novel. Improving speech intelligibility with directional microphone arrays, for example, is known.
For a directional microphone array aimed at a talker in the presence of diffuse noise, an increase in array directivity yields an increase in talker-to-noise ratio (TNR). This increase in TNR can lead to an increase in speech intelligibility for a user listening to the array output. Excluding some complexities discussed later, increasing array directivity increases speech intelligibility gain.
Consider the four microphone array 10 in FIG. 1 located on the head of a user. In a prior art beamforming approach, the arrays are designed assuming the individual microphone elements are located in the free field. An array for the left ear is created by beamforming the two left microphones 20 and 21. The right ear array is created by beamforming the two right microphones 22 and 23. Well-established free field beamforming techniques for such simple, two-element arrays can create hypercardioid free-field reception patterns, for example. Hypercardioids are common in this context, as in the free-field they produce optimal TNR improvement for a two element array for an on-axis talker in the presence of diffuse noise. Arrays such as array 10 when designed for free field performance may not meet performance criteria when placed on the head because of the acoustic effects of the head on sound received by the microphone elements that make up the array. Further, arrays such as array 10 may not provide sufficiently high directivity to significantly improve speech intelligibility.
Head-mounted arrays, especially those with high directivity, can be large and obtrusive. An alternative to head-mounted arrays are off-head microphone arrays, which are commonly placed on a table in front of the listener or on the listener's torso, after which the directional signal is transmitted to an in-ear device commonly employing hearing-aid signal processing. Although these devices are less obtrusive, they lack a number of important characteristics. First these devices are typically monaural, transmitting the same signal to both ears. These signals are devoid of natural spatial cues and the associated intelligibility benefits of binaural hearing. Second, these devices may not provide sufficiently high directivity to significantly improve speech intelligibility. Third, these devices do not rotate with the user's head and hence do not focus sound reception toward the user's visual focus. Also, the array design may not take into account the acoustic effects or the structure that the microphones are mounted to.
White noise gain (WNG) describes the amplification of uncorrelated noise by the array processing and is well defined in the art. WNG is essentially the ratio of total array filter energy to received pressure through the array for an on-axis source. This quantity describes how array losses due to destructive interference will increase the system noise floor, for example. A simple hypercardioid array is a lossy array which may yield too much self-noise when equalized for flat on-axis response. Failure to consider the WNG of a particular array design can result in a system with excessive self-noise.